In the related-art, a rotational-angle detection device for detecting a rotational angle of a rotary shaft by means of a magnetic sensor is known. For example, Patent Literature 1 proposes a rotational-angle detection device including a magnetic member fixed to a rotary shaft and magnetized as an N pole and an S pole at predetermined intervals in a peripheral direction of the rotary shaft, and a sensor part including two MR sensors arranged so as to oppose the magnetic member to detect a change in a magnetic field caused by the N pole and the S pole at positions 180-degree symmetrical about an axial center of the rotary shaft in the peripheral direction of the rotary shaft. Each of the two MR sensors includes two magnetoresistance effect devices (MR devices) presenting such a characteristic that an electric resistance changes depending on the magnitude of a magnetic flux density when the MR sensors are respectively influenced by an external magnetic field. The two MR devices are serially connected to each other, a predetermined voltage is applied to both ends thereof, and a voltage at a connection point between both of the MR devices is output in the MR sensor.
When a relative positional relationship between the center axis of the rotary shaft and the sensor part is misaligned in the radial direction of the rotary shaft (hereinafter the misalignment is referred to as shaft misalignment), the rotational angle cannot be accurately detected. Thus, the rotational-angle detection device according to Patent Literature 1 cancels the detection error in the rotational angle caused by the shaft misalignment by using an average of results obtained from the outputs of the two MR sensors to acquire the rotational angle.